The electrolysis of aqueous salt solutions in permselective membrane electrolytic cells is well documented in the literature as are the advantages and disadvantages associated therewith. It is known, for example, that when employing such cells for the production of sodium hydroxide (caustic soda) and chlorine from sodium chloride brines, the current efficiency decreases as the concentration of sodium hydroxide in the catholyte increases. It is of course desirable to be able to produce relatively concentrated caustic soda solutions without recourse to separate concentration procedures and with acceptable current efficiencies. One method for attaining this end is disclosed in U.S. Pat. No. 4,057,474 wherein there is described a process involving the flow of catholyte sodium hydroxide solutions exiting from one cell to the catholyte compartment of a succeeding cell in a bank. This "series catholyte flow" process results in the recovery of relatively concentrated sodium hydroxide solutions directly from the last of the cells in a series and at the same time, the average current efficiency for the cells in the series is well within the acceptable range. German Offenlegungsschrift 2,437,783 and U.S. Pat. No. 4,076,603 also describe a series catholyte flow process.
While such series catholyte flow results in substantial improvement in current efficiency, certain problems have become apparent in operating a bank of cells with series flow. Some means must be provided for transferring the catholyte from one cell to another. This can be accomplished with conventional pumps, but this requires additional equipment and, depending on how the cells are serially connected, a breakdown of just one pump could conceivably disrupt the whole operation. While the use of gravity flow has been postulated, commercial operation involves a large number of cells in a bank, and thus gravity becomes impractical because of design and/or operational problems associated with the necessity that succeeding cells connected together in series catholyte flow must necessarily be at lower elevations in order for flow from one cell to the next to occur. Additionally, when succeeding cells are at a different voltage, as in a filter press type bipolar cell stack or among individual monopolar cells connected via a series electrical circuit, the transfer of catholyte from one cell to another should be done in such a manner as to insure electrical isolation of one cell from another.
It is an object of this invention to provide a process for operating a bank of electrolytic cells connected for series catholyte flow. It is a further object of this invention to provide a process for transferring catholyte from one cell to another which does not require external pumping means and which serves to substantially isolate the cells electrically. These and other objects will become apparent from the description which follows.